1. Field of the Invention
The present invention relates to an apparatus and method for estimating a location of a Mobile Station (MS) in a wireless communication system. More particularly, the present invention relates to an apparatus and method for reducing a location estimation error caused by a repeater.
2. Description of the Related Art
In order to support a higher transmission rate than a 3rd Generation (3G) mobile communication system and extend service coverage, a new 4th Generation (4G) mobile communication system is being developed. Many research institutes and enterprises of advanced countries are promoting the development of technology for future 4G-standardization.
A 4G mobile communication system operating in a high-frequency domain is limited in its service coverage because of a high path loss. In order to address this problem, a multi-hop signal forward scheme has been investigated. A multi-hop technology can reduce a path loss and make a high-speed data communication possible by relaying data using a relay, and can extend service coverage by forwarding a signal to a Mobile Station (MS), even if the MS is far away from a Base Station (BS).
As such, a multi-hop relay system requires one or more relays for relaying data between a transmitter and a receiver. In a cellular system, a relay can be a separate device distinguished from a BS and an MS. However, an MS can serve as a relay for a different MS. In the multi-hop relay system, a communication between two nodes is implemented through a transmitter-relay, relay-relay, relay-receiver radio link and the like.
Typically, a multi-hop relay technology can be divided into an amplify-and-forward scheme and a decode-and-forward scheme. The amplify-and-forward scheme is a scheme in which a relay amplifies a Radio Frequency (RF) signal received from a transmitter and relay-forwards the amplified signal to a receiver. The amplify-and-forward relay may be referred to in the alternative as a “repeater.” The decode-and-forward scheme is a scheme in which a relay demodulates and decodes a received signal, again modulates and encodes the signal, and relay-forwards the signal to a receiver. The decode-and-forward relay may be referred to in the alternative as a “Relay Station (RS).” The description below is made describing an amplify-and-forward relay (i.e., a repeater) as an example.
Due to an increase of demand for a variety of services, a Location Based Service (LBS) has been developed in various forms. The LBS is, for example, a service of estimating a location of a user who is under emergency (i.e., disaster) circumstances, a service of estimating a location of a user (that is, an MS) and guiding the user to a restaurant and the like.
A Location Detection Technology (LDT) for the LBS has been widely known as a cell IDentifier (ID) technique, an Angle Of Arrival (AOA) technique, a Time Of Arrival (TOA) technique, a Time Difference Of Arrival (TDOA) technique, an Assisted-Global Positioning System (A-GPS) technique and the like. Among the LDT technologies, the A-GPS technique is the one mostly used due to it having the highest degree of accuracy. However, the A-GPS technique requires a minimum of four GPS satellites and thus, in a shadow area where less than four satellites are available, a hybrid GPS technology using TDOA or cell ID is used. That is, hybrid GPS can use either TDOA or cell ID according to requirements of a desired service or number of BSs that can be secured by a user and the like.
In a metropolitan area, many shadow areas can exist between a BS and an MS and thus, many repeaters may be used. When a TOA or TDOA technology is used for LBS, it requires a minimum of three BSs. However, when a signal from a BS is received by an MS through a repeater, a time delay of the received signal through the repeater is different then a time delay of the signal received directly from the BS. This difference in the time delay results in a location estimation error.
FIG. 1 illustrates that an MS receives signals from three BSs in a conventional wireless communication system.
When a signal from each BS is received via a real path through a repeater and not a desired path, a time delay occurs because of the repeater. Thus, an MS cannot calculate an accurate propagation delay (i.e., TDOA, TOA, etc.). That is, because the MS calculates TDOA considering a received signal as directly received from a BS, a location estimation error can occur.
In addition, when a signal from a BS is received by an MS through a plurality of repeaters, the MS cannot be aware if a signal is received through a repeater and thus it cannot accurately determined if the measured time delays correspond to a path though a repeater. Moreover, numerous repeaters are installed in a metropolitan area. These numerous repeaters deteriorate location estimation performance, which causes a problem when using a LDT.